Due to, among other things, their small size, high quality factor (Q) values, and very low insertion losses at microwave frequencies, particularly those above 1.5 Gigahertz (GHz), Bulk Acoustic Wave (BAW) filters have become the filter of choice for many modern wireless applications. In particular, BAW filters are the filter of choice for many 3rd Generation (3G) and 4th Generation (4G) wireless devices. For instance, virtually all Long Term Evolution (LTE) compatible mobile devices operating in LTE frequency bands above 1.9 GHz utilize BAW filters. For mobile devices, the low insertion loss of the BAW filter provides many advantages such as, e.g., improved battery life, compensation for higher losses associated with the need to support many frequency bands in a single mobile device, etc.
One example of a conventional BAW resonator 10 is illustrated in FIG. 1. In this example, the BAW resonator 10 is, in particular, a Solidly Mounted Resonator (SMR) type BAW resonator 10. As illustrated, the BAW resonator 10 includes a piezoelectric layer 12 (which is sometimes referred to as a piezoelectric plate) between a bottom electrode 14 and a top electrode 16. In this example, the top electrode 16 includes a high (electrical) conductivity layer 16A and a high (acoustic) impedance layer 16B. An enlarged illustration of the top electrode 16 is illustrated in FIG. 2. Since the BAW resonator 10 is a SMR type BAW resonator 10, the BAW resonator 10 also includes an acoustic reflector 18 (e.g., a Bragg reflector) that includes layers 20-28 of alternating high and low acoustic impedance. In this example, the BAW resonator 10 also includes a Border (BO) ring around the periphery of the top electrode 16 within what is referred to herein as a BO region 30 of the BAW resonator 10. The BO region 30 is the peripheral region of an active region 32 of the BAW resonator 10. As used herein, the active region 32 of the BAW resonator 10 is the region of the BAW resonator 10 that is electrically driven. An outer region 34 of the BAW resonator 10 is the region of the BAW resonator 10 that is outside of the active region 32.
In operation, acoustic waves in the piezoelectric layer 12 within the active region 32 of the BAW resonator 10 are excited by an electrical signal applied to the bottom and top electrodes 14 and 16. The active region 32 is the region of the BAW resonator 10 that is electrically driven. In other words, the active region 32 is the region of the BAW resonator 10 consisting of, in this example, the bottom electrode 14, the top electrode 16, the portion of the piezoelectric layer 12 between the bottom and top electrodes 14 and 16, and the portion of the acoustic reflector 18 below the bottom electrode 14. Conversely, the outer region 34 of the BAW resonator 10 is a region of the BAW resonator 10 that is not electrically driven (i.e., the area outside of the active region 32). The frequency at which resonance of the acoustic waves occurs is a function of the thickness of the piezoelectric layer 12 and the mass of the bottom and top electrodes 14 and 16. At high frequencies (e.g., greater than 1.5 GHz), the thickness of the piezoelectric layer 12 is only micrometers thick and, as such, the BAW resonator 10 is fabricated using thin-film techniques.
Ideally, in order to achieve a high Q value, the mechanical energy should be contained, or trapped, within the active region 32 of the BAW resonator 10. The acoustic reflector 18 operates to prevent acoustic waves from leaking longitudinally, or vertically, from the BAW resonator 10 into a substrate 36. Notably, in a Film Bulk Acoustic Resonator (FBAR) type BAW resonator, an air cavity is used instead of the acoustic reflector 18, where the air cavity likewise prevents acoustic waves from escaping into the substrate 36.
One issue that arises with the conventional BAW resonator 10 of FIG. 1 is that thickening of the top electrode 16, and in particular the high conductivity layer 16A, is desirable in order to gain conductivity; however, thickening of the top electrode 16, and in particular the high conductivity layer 16A, changes the characteristics of the BAW resonator 10 and degrades the performance of the BAW resonator 10. Thickening of the top electrode 16 results in mass loading of the BAW resonator 10, which in turn changes the characteristics (e.g., resonant frequency) of the BAW resonator 10. Additionally, thickening of the top electrode 16 drives acoustic energy into the highly conductivity layer 16A (e.g., Aluminum (Al)), which can be acoustically lossly and drive down the Q factor of the BAW resonator 10. Finally, thickening of top electrode metals can also change acoustic wave confinement which can drop Q factor.
In light of the discussion above, there is a need for a BAW resonator architecture that provides improved metal electrode conductivity while avoiding the issues described above.